1. Field of the Invention
This invention provides clad magneto-optical waveguides of aluminum garnet suitable for use at high temperatures and at visible, blue, infra-red and ultra-violet wavelengths. These structures comprise a waveguiding body of magneto-optically active crystalline aluminum garnet of a high refractive index, clad with crystalline aluminum garnet of a lower refractive index. They can be in the form of fibers, slabs, channels, ribs, or any of the typical optical waveguide structures. Their applications include optical isolating and magnetic field sensing elements.
2. Description of the Prior Art
High temperature waveguides are commonly made of sapphire, a crystal form of the high melting oxide Al.sub.2 O.sub.3 (melting point 2054.degree. C.). Optical waveguides of sapphire have significant optical loss due to the lack of a suitable cladding material. A metal overcoat is used to protect such waveguides from the environment, but the transmission efficiency of this structure is low. A low loss optical waveguide requires a higher refractive index core surrounded by a lower refractive index cladding, and this is not provided in the metal-clad sapphire core waveguides.
P. J. Chandler et al. [Electron. Lett. 25, 985 (1989)] have used an ion-implantation technique to produce a slab waveguide in the aluminum garnet (Y,Nd).sub.3 Al.sub.5 O.sub.12. This ion-implantation technique, unlike the technique of the present invention, makes use of the displacement of atoms in the crystal from their usual positions in the crystal lattice to generate regions of a small refractive index change. This ion-implantation technique is not suitable for use in high temperature waveguides, since the crystal structure will relax to its equilibrium state after exposure to high temperature.
Iron garnets are important magneto-optical materials because of their high Faraday rotation. Yttrium iron garnet (YIG), for example, has a Faraday rotation of about 0.1 deg/.mu.m at visible wavelengths. Bismuth-substituted iron garnet also has large Faraday rotation [D. M. Gualtieri et al., J. Appl. Phys. 57, 3879 (1985)]. A magneto-optical sensor using a yttrium iron garnet (YIG) single crystal at 1300 nm has been described by Zook et al. [Appl. Optics 28, 1991 (1989)]. Their device consists of a YIG rod, 5 mm diameter by 6.3 mm length, mirrored at one end, which is coupled through a polarizer and a gradient index (GRIN) rod to an input and output fiber. Waveguide optical isolators of iron garnet have been described by E. Pross et al.[Appl. Phys. Lett. 52, 682 (1988)]; R. Wolfe et al. [Appl. Phys Lett. 56, 426 (1990)]; and R. Wolfe et al. [Appl. Phys. Lett 57, 960 (1990).
The principal disadvantage in the use of iron garnet in magneto-optical waveguides is the limited temperature range of performance. Iron garnets are ferrimagnets which maintain their ideal magnetic properties only up to a definite temperature limit, i.e. the Curie temperature. A representative iron garnet, YIG, has a Curie temperature of 556K (283.degree. C.). Bismuth substitution for yttrium increases the Curie temperature by 38K for every formula unit atom [P. Hansen et al., Phys. Rev. B27, 6608 (1983)], allowing operation of waveguide isolators and sensors of this material to a maximum temperature of about 300.degree. C. There are many applications which require operation above this temperature
Another major problem with the use of iron garnet, and to a somewhat greater extent of bismuth iron garnets, in magneto-optical waveguides is their high optical absorption at visible and shorter wavelengths. The figure of merit of these materials, which is defined as the ratio of the optical rotation to the optical absorption, decreases at short wavelengths. Bismuth substituted garnets are generally useful as optical isolators only at long wavelengths, such as 1300 and 1500 nm, where the figure of merit is of the order of hundreds of deg/dB.